GRB 050502B optical afterglow: a jet-break at high redshift⋆
Max-Planck-Institut für extraterrestriche Physik,
2 Osservatorio Astronomico di Trieste, Istituto Nazionale di Astrofisica, via G.B. Tiepolo 11, 34131 Trieste, Italy
3 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
4 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
5 Osservatorio Astronomico di Brera, Istituto Nazionale di Astrofisica, via Brera 28, 20121 Milano, Italy
6 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
7 Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
8 University College of Dublin, Science Centre, School of Physics, Belfield, Dublin 4, Ireland
9 Princeton University, Department of Astrophysical Sciences, Peyton Hall, Princeton, NJ 08544-1001, USA
Accepted: 5 October 2010
Aims.Swift GRB 050502B is well known for the very bright flare displayed in its X-ray light curve. Despite extensive studies, however, the optical light curve has never been discussed and its redshift is unconstrained. Possible correlations between optical and X-ray data are analysed.
Methods. Photometric data from TNG in the R and I bands were used to compare the optical afterglow with the X-ray light curve. The HyperZ package and a late-time VLT host observation were used to derive redshift estimates.
Results. The I-band afterglow decay followed a power law of index α = 2.1 ± 0.6, after a late break at ~1.3 × 105 s. The R − I colour is remarkably red and the broadband spectral index βOX = 0.9 ± 0.1 is consistent with the X-ray spectral slope βX. Although a photometric redshift of z > 4 is the most conservative result to consider, a photometric redshift of z = 5.2 ± 0.3 is suggested with no extinction in the host, based on which an isotropic energy Eγ,iso = (3.8 ± 0.7) × 1052 erg and a jet opening angle θ ~ 3.7° are subsequently derived.
Conclusions. The combined X-ray and optical data suggest an achromatic break, which we interpret as a jet-break. The post jet-break slope roughly obeys the closure relation for the jet’s slow cooling model. Because of the afterglow’s very red colour, in order for the redshift to be low (z < 1), extinction must be significantly high if present in the host. Since the optical-to-X-ray index is consistent with the X-ray spectrum, and there is no XRT evidence for excess NH, GRB 050502B was likely at high redshift.
Key words: gamma-ray burst: general / gamma-ray burst: individual: GRB 050502B
Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (programme AOT11-59) and with ESO Telescopes at the La Silla Paranal Observatories under programme ID 177.A-0591.
© ESO, 2011